Polyesters are important articles of commerce. Generally, they are prepared by the following processes:
1. Esterification of a hydroxycarboxylic acid.
2. Esterification of a diacid and a diol.
3. Ester interchange with alcohol.
4. Ester interchange with ester.
5. Esterification of acid chlorides.
6. Lactone polymerization.
Ester interchange reactions are commercially important since (a) methyl esters are easier to purify than the diacids themselves, and (b) by-product methanol is easily removed by distillation. For example, poly(ethylene terephthalate) is made commercially by a process which comprises:
(i) reacting dimethylterephthalate with ethylene glycol at a temperature below 200.degree. C. to produce 2-hydroxyethylterephthalate, and
(ii) producing the polymer by raising the temperature above the melting point of the polymer, ca. 260.degree. C. As can be seen, polymer formation requires a fairly elevated temperature.
The carbonylation of aryl bromides and iodides using Pd(PPh.sub.3).sub.2 X.sub.2 as a catalyst to produce monomeric esters is known. Until this invention, such carbonylations have not been used to produce polyesters.